Logic Gates - Insights

Results of a Poll of IHS Engineering360 subscribers

Types of Logic Gates
Logic gates are circuits with electronically controlled switches that combine digital signals according to Boolean algebra. In binary math, bits have only two possible values: 0 (off, false) and 1 (on, true). Consequently, there are three basic logic gates from which all other combinatorial logic functions are generated.
Inverters, or NOT gates, have only one input and reverse the logic state. A true input produces a...
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Q & A on Logic Gates

We asked our users for their input on Logic Gates.
Here are the results of 92 users
familiar with Logic Gates.

Do you know of any disastrous mistakes that occurred due to the incorrect usage of logic gates?

6 answers

Answers:

Radar simulator design that tried to use cascaded gates to meet delay requirements for data transfer. The product would only work over a very narrow temp range and the company that designed the product was so late to deliver the product it lost the contract.

~David D'Andrea, Design Engineer, Getzville, NY

Mixing families of different voltage ratings. For example, an open collector or open drain line can be driven by one family and input to another and will work intermittently if the voltage ranges overlap, but it's not correct.

~John D, Research & Development, London, Canada

Bought and used logic gates from a manufacturer that has specs for -40 to 85 deg C, in the environment that required -55 to 125 deg C specs.

~Component Engineer, Peterborough, Canada

No, I have managed to avoid disastrous mistakes by careful and thoughtful engineering practices.

Is there any advice you want to share with users to help them avoid common errors in selection or usage of logic gates?

9 answers

Answers:

Watch out for CMOS TTL compatibility problems (they can often be solved with a small: say 47 Ohm resistor). Make sure that the data inputs change on the non active edge of the clock. Make sure you understand the difference between level sensitive and edge sensitive design.

~R. A. B, Design Engineer, Hemel Hempstead, UK

Do not choose through hole non RoHS parts(they are going obsolete). Pay special attention to power dissipation, temperature range of the devices (Similar logic gates from different suppliers may have different temperature and power dissipation specs.)

~Component Engineer, Peterborough, Canada

Pay strict attention to propagation delays and path lengths. Keep things isolated between analog circuitry and digital. Pay attention to how power and ground are routed to minimize glitches and crosstalk.

~Gerard R, Engineering Consultant, Fremont, CA

The fastest gate is not always the best choice for a design. Study the speed and delay requirements for each application and pick the family that meets those requirements.

~David D'Andrea, Design Engineer, Getzville, NY

Be careful when using them in mixed signal applications. They can inject a lot of noise into analog sections.

~John D, Research & Development, London, Canada

First build a truth table and then build up with LOGO Soft (PC program), the circuit.

Do you have any advice for people relative to buying or using logic gates?

8 answers

Answers:

Make sure that the parts you selected are relatively new, not going to be obsolete in the near future. If the life cycle of your product is 10 years plus, ensure that the supply of the parts selected can last at least 10 years plus.

~Component Engineer, Peterborough, Canada

Look at the full specification including voltage rail tolerance, supply rail current and input signal logic thresholds. Work out what logic speed you actually need.

~Steve H, Design Engineer, Portsmouth, England

If the design needs more than a few chips, buy an FPGA dev kit and evaluate.

~R. A. B, Design Engineer, Hemel Hempstead, UK

Pick the lowest voltage family that the design will support.

~David D'Andrea, Design Engineer, Getzville, NY

Supply voltage should be low therefore, buy cmos gates.

~Kunal J, Student, Faridabad, India

In East Europe (I am from Romania) it's a big problem with authenticity of many electronic components. Too many fakes... Just using consecrated providers is possible to have certitude of authenticity. Any way, before purchase, search data sheet of manufacturer (some time and by year of manufacturing of the same manufacturer) especially in case of replacements of originals with other type or other manufacturer.

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